1. Field of the Invention
This invention relates to a glass substrate for a magnetic disk, and more particularly to a glass substrate of this kind which has a predetermined composition and is formed with protrusions on a surface thereof by irradiating a laser beam on the surface to form texture.
2. Description of the Related Art
In general, a fixed magnetic disk unit employs a mechanism called a CSS (Contact Start Stop) system in which a magnetic head is in contact with a disk surface when the disk is stationary and is brought into sliding contact with the same when the disk is started and stopped.
In the CSS system, to prevent stiction from occurring when the disk is started and stopped as well as to reduce friction which will occur on such occasions, so-called texture, i.e. a suitably finely-roughened surface formed by protruded portions and recessed portions (which may be formed by protruded portions alone) is provided on the disk. The texture is provided on all or part of a main surface of the disk, where a magnetic layer is formed. Assuming that the texture is formed merely on part (CSS zone) of the main surface, the magnetic head is moved to the CSS zone at proper timing when the disk is brought into a CSS operation. Further, when the power is turned off during rotation of the disk, the magnetic head is also moved to the CSS zone.
Particularly, when the texture is formed on part of the main surface, the remaining part can preserve smoothness similar to that of a mirror surface, which enables the magnetic head to be floated at a lower position. Therefore, the partially-formed texture is suitable for increasing the recording density (packing density) of the magnetic disk unit.
For a substrate of this disk, an Al--Mg alloy substrate plated with Ni--P, i.e. a so-called aluminum substrate, has been widely employed. To provide texture on the aluminum substrate, a method of forming concentric scratches on the substrate by using an abrasive tape has been widely in practice. However, this method is difficult to meet the requirements of prevention of stiction and reduction of friction at the same time when a further lower floating position is demanded of the magnetic head.
To solve this problem, various methods have been proposed. U.S. Pat. Nos. 5,062,021 and 5,108,781, for instance, disclose a process of forming pits comprised of recesses and annular projections surrounding the recesses on a surface of a metal of an aluminum substrate so as to reduce stiction. The above two U.S. Patents also disclose a method of generating a required surface roughness by using a Nd:YAG laser.
Further, a glass substrate has more advantageous properties over the aluminum substrate in that a surface of the former can be more easily smoothed by polishing than one of the aluminum substrate, the former is more rigid than the latter assuming that they have an identical thickness, the former has an excellent shock resistance, etc.
The above-mentioned technique of forming texture is more important to the glass substrate since its surface can be easily smoothed.
As the method of forming texture on the glass substrate, there has been proposed one based on irradiation of a laser beam. Japanese Laid-Open Patent Publication (Kokai) No. 4-311814, for instance, discloses a method of forming texture on a glass substrate by irradiating a pulsed laser beam on a back plate from a reverse side of the glass substrate arranged at a predetermined distance from the back plate, thereby causing molten fine particles scattered from the surface of the back plate to collide with a surface of the glass substrate.
Japanese Laid-Open Patent Publication (Kokai) No. 7-182655 discloses a method of forming texture on a brittle material, such as a glass, and describes that it is possible to carry out texture processing on a brittle substrate, such as a glass substrate, which has a thermal shock limitation, by controlling a particle fluence of irradiation energy to a suitable value lower than the thermal shock limitation. Below an energy fluence limitation (thermal shock limitation) at which transition is drastic, the energy fluence of the pulsed laser beam has no influence on the glass substrate or merely forms a bump without causing any damage. In the case of a glass disk having a stress of surface compression, almost all of such a bump is protruded upward from a nominal surface, which is useful in reducing stiction of a data storage disk.
According to the publication, this technique of texture processing is capable of forming texture on a surface of a glass substrate at a low cost with excellent controllability. Further, it is also possible to form texture on a CSS region alone of the glass substrate.
Further, Japanese Laid-Open Patent Publication (Kokai) No. 8-147687 discloses a method of irradiating a pulsed laser beam on an aluminum alloy substrate or a glass substrate with a laser output power of 500 mW or less, over 5 .mu.sec or shorter per one irradiation, by forming a spot of the laser beam on the irradiated surface, which has a diameter of 5 .mu.m or smaller and moved at a speed of 1 m/sec or higher, to thereby form protrusions at irradiated portions of the surface of the substrate.
Japanese Laid-open Patent Publication (Kokai) No. 7-182655 describes that a laser beam is employed which has a certain range of an optical transmittance of a laser beam energy pulse against the glass. However, there is no description concerning the relation of the laser beam with a composition of the glass, and still less calculation of the height of protrusions formed on the surface of the glass. Further, as to a wavelength of the laser beam, only a value of 10.6 .mu.m is disclosed but no other wavelength is mentioned. Therefore, there is no disclosure of a wavelength of the laser beam suitable for attaining provision of a uniform bump by a laser beam at a low output power.
By the way, it is known that in general, in a texture part of the magnetic disk, so long as a ratio of an area of projections of the texture to a total area of the surface of the magnetic disk is identical, projections of the texture each having a smaller diameter, i.e. projections of the texture with smaller space intervals are more suitable for a lubricant to take effect, thereby improving abrasion resistance of the magnetic disk (1) Hiroshi Tani et al, Japan Tribology Society, Draft Collection of Tribology Meeting at Kanazawa, October, 1994, p. 153; (2) H. Ishihara et. al, "Wear", Vol. 172 (1994) p. 65!. Therefore, it is preferred that each protrusion of the texture has a smaller diameter than one disclosed in an example of Japanese Laid-Open Patent Publication (Kokai) No. 7-182655 (in which each protrusion of texture having a diameter of 30 .mu.m).
However, in the example disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 7-182655, if the number of apertures for lenses is increased and the diameter of a spot of an irradiated laser beam is reduced, there arises a problem that protrusions of texture formed suffer from an undesired variation in size. Presumably, since the number of apertures for lenses is increased and the diameter of each laser beam is reduced, undulations of the surface of the disk causes the diameter of the spot of each laser beam to vary from area to area of the surface of the disk. Such an undesired variation in diameter of each projection of texture on the disk surface has an adverse effect on CSS characteristics, gliding characteristics, etc. of the disk.
Further, the technique described in Japanese Laid-Open Patent Publication (Kokai) No. 8-147687 has no reference to a relation between the composition of a glass and an optical absorption coefficient of a glass substrate when glass is used as a substrate of a magnetic disk. Therefore, it is impossible to determine a value of the optical absorption coefficient, which is necessary for forming protrusions on the glass substrate with accuracy and efficiency. Moreover, the publication does not contains any suggestion on the relation between the absorption coefficient and the height of projections nor the relation between the absorption coefficient and the output power of a laser beam.